The present invention relates to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
For example, U.S. Pat. No. 5,493,971 discloses wet-plate constructions that extend the benefits of ablative laser imaging technology to traditional metal-based plates. Such plates remain the standard for most of the long-run printing industry due to their durability and ease of manufacture. As shown in FIG. 1, a lithographic printing construction 100 in accordance with the xe2x80x2971 patent includes a grained-metal substrate 102, a protective layer 104 that can also serve as an adhesion-promoting primer, and an ablatable oleophilic surface layer 106. In operation, imagewise pulses from an imaging laser (typically emitting in the near-infrared, or xe2x80x9cIRxe2x80x9d spectral region) interact with the surface layer 106, causing ablation thereof and, probably, inflicting some damage to the underlying protective layer 104 as well. The imaged plate 100 may then be subjected to a solvent that eliminates the exposed protective layer 104, but which does no damage either to the surface layer 106 or the unexposed protective layer 104 lying thereunder. By using the laser to directly reveal only the protective layer and not the hydrophilic metal layer, the surface structure of the latter is fully preserved; the action of the solvent does no damage to this structure.
A related approach is disclosed in published PCT Application Nos. US99/01321 and US99/01396. A printing member in accordance with this approach, representatively illustrated at 200 in FIG. 2, has a grained metal substrate 202, a hydrophilic layer 204 thereover, an ablatable layer 206, and an oleophilic surface layer 208. Surface layer 208 is transparent to imaging radiation, which is concentrated in layer 206 by virtue of that layer""s intrinsic absorption characteristics and also due to layer 204, which provides a thermal barrier that prevents heat loss into substrate 202. As the plate is imaged, ablation debris is confined beneath surface layer 208; and following imaging, those portions of surface layer 208 overlying imaged regions are readily removed. Because layer 204 is hydrophilic and survives the imaging process, it can serve the printing function normally performed by grained aluminum, namely, adsorption of fountain solution.
Both of these constructions rely on removal of the energy-absorbing layer to create an image feature. Exposure to laser radiation may, for example, cause ablationxe2x80x94i.e., catastrophic overheating-of the ablated layer in order to facilitate its removal. Accordingly, the laser pulse must transfer substantial energy to the absorbing layer. This means that even low-power lasers must be capable of very rapid response times, and imaging speeds (i.e., the laser pulse rate) must not be so fast as to preclude the requisite energy delivery by each imaging pulse.
The present invention obviates the need for substantial ablation as an imaging mechanism, combining the benefits of simple construction, the ability to utilize traditional metal base supports, and amenability to imaging with low-power lasers that need not impart ablation-inducing energy levels. In preferred embodiments, the invention utilizes a printing member having a topmost layer that is ink-receptive and a hydrophilic metal substrate. The topmost layer does not significantly absorb imaging radiation, but an intermediate layer disposed between the topmost layer and the metal substrate does absorb imaging radiation. In one version, in response to an imaging pulse, the absorbing layer debonds from the surface of the adjacent metal substrate; in another version, an interior split is formed within the absorbing layer, facilitating removal of the portion of that layer above the split. In neither case does the absorbing layer undergo substantial ablation.
It must be stressed that it is ordinarily impractical or even impossible to image, by ablation, constructions in which an absorbing layer directly overlies the metal substrate. This is because because the thick metal substrate acts as a heat sink, drawing laser energy needed to heat the absorbing layer to achieve imaging. Because ablation is not involved as an imaging mechanism in the present invention, however, this condition is avoided. Sufficient energy is concentrated in the upper portions of the absorbing-layer thickness to cause debonding notwithstanding heat transport into the metal substrate. It is also possible to create an absorber gradient within the absorbing layer, with the absorber concentration diminishing from the top of the layer to the bottom, so that the surface in contact with the metal substrate has very little absorber. This concentration gradient further discourages transfer of heat to the metal substrate while preserving sufficient overall absorption and heating to effect interfacial debonding. Indeed, some transfer of heat to the metal substrate (as well as to an overlying layer, when present) is desirable to avoid unintended ablation of the absorbing layer, which can result in production of unwanted volatile debris.
In use, the printing member is selectively exposed to laser radiation in an imagewise pattern. Where the printing member has received laser exposurexe2x80x94that is, where the substrate and absorbing layer have been detached from each otherxe2x80x94remnants of the absorbing layer and the overlying layer (or layers) is readily removed by post-imaging cleaning (see, e.g., U.S. Pat. Nos. 5,540,150; 5,870,954; 5,755,158; and 5,148,746) to produce a finished printing place.
Accordingly, layers that would otherwise undergo complete destruction as a consequence of ablation imaging are retained in the present constructions, and serve as highly durable layers that participate in the printing process. Key to the present invention, then, is irreversible detachment between layers caused by heating, without ablation, of a radiation-absorptive layer, and an absorber concentration gradient that prevents excessive energy dissipation from the absorbing layer.
The plates of the present invention are xe2x80x9cpositive-workingxe2x80x9d in the sense that inherently ink-receptive areas receive laser output and are ultimately removed, revealing the hydrophilic layer that will reject ink during printing; in other words, the xe2x80x9cimage areaxe2x80x9d is selectively removed to reveal the xe2x80x9cbackground.xe2x80x9d Such plates are also referred to as xe2x80x9cindirect-write.xe2x80x9d
It should be noted that, as used herein, the term xe2x80x9cplatexe2x80x9d or xe2x80x9cmemberxe2x80x9d refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution; suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.
Furthermore, the term xe2x80x9chydrophilicxe2x80x9d is used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water for conventional ink systems, aqueous and non-aqueous dampening liquids, and the non-ink phase of single-fluid ink systems. Thus, a hydrophilic surface in accordance herewith exhibits preferential affinity for any of these materials relative to oil-based materials.